This invention relates generally to the production of phosphor screens for color picture tubes and particularly to a unitary light shield for use in a lighthouse used to produce such screens.
A color picture tube includes a screen composed of triads of different phosphors which emit different colored light when excited by electrons. Typically, the screen is composed of alternating stripes or dots of phosphors which respectively emit red, green and blue light. Positioned between the screen and the electron gun from which the exciting electrons emanate is a color selection electrode, commonly called a shadow mask. The shadow mask assures that the electron beams excite phosphor stripes of the proper color.
During the production of the phosphor screen, the entire inside surface of the panel is coated with a slurry which consists of one of the phosphors mixed in photosensitive material. The shadow mask is then inserted into the panel and the assembly is placed onto a lighthouse which contains a light source. Light from the light source passes through the apertures in the shadow mask and exposes some of the phosphor. The shadow mask is then removed and the unexposed phosphor is washed away leaving only the exposed phosphor. This process is then repeated for the remaining two colors of phosphors.
During the application of the slurry to the inside surface of the panel, the slurry is applied while the panel is rotating to spread the slurry over the entire inside surface. The excess slurry is then poured from the panel and thus some of the slurry adheres to the sidewalls of the panel. During the light exposure process, a plurality of light shields are used in an effort to direct the light only to those portions of the screen where the retention of the phosphor material is desired. However, problems sometimes arise because the penumbra light exposes some of the phosphor material on the sides of the panel irrespective of the presence of the light shield. Accordingly, it ordinarily is necessary to trim the sides of the panels to remove the exposed slurry to avoid difficulties in the operation of the finished tube.
The instant invention overcomes this difficulty by the provision of a unitary light shield which effectively blocks the penumbra light from the panel sidewalls to substantially reduce, or eliminate, exposure of the slurry on the panel sidewalls, thereby substantially reducing the need for trimming exposed phosphors from the sidewalls.